Trump may declare a national emergency in the border wall battle. Here’s what that means.

President Trump spent the week visiting the U.S.-Mexico border and hinting he might declare a national emergency to build his long-discussed wall. On Friday, he put the idea on hold, strongly counseling congressional Democrats to rethink opposing the wall’s funding.

“What we’re not looking to do right now is national emergency,” he said Friday afternoon. “I’m not going to do it so fast.”

Despite this temporary reprieve, the partial government shutdown drags on.

For Trump, the issue is twofold, according to Gerald S. Dickinson, assistant professor at the University of Pittsburgh School of Law: To what extent is illegal immigration a national crisis, and is it unusual or extraordinary for individuals to come across the southern border?

“We’ve been dealing with this for many years through congressional legislative processes. Trump is using this power as an alternative lever to achieve a policy goal he can’t reach through Congress,” he said.

So what is a “national emergency,” and what does it mean for Trump’s border wall?

What is a national emergency?

In 1976, Congress passed the National Emergencies Act, which permits the president to pronounce a national emergency when he considers it appropriate. The act offers no specific definition of “emergency” and allows a president to declare one entirely at his or her discretion.

It has historically been invoked during unusual or extraordinary threats to the United States, according to Dickinson, like in response to the threat of North Korean nuclear warfare or amid the 1979 Iran hostage crisis.

By declaring a national emergency, the president avails himself of dozens of specialized laws. Some of these powers have funds he otherwise could not access.

Under current law, emergency powers lapse within a year unless the president renews them. A national emergency can be re-declared indefinitely and, in practice, is done frequently. There have been 58 pronounced under the National Emergencies Act, of which 31 are still in effect.

When have they been declared in the past?

Presidents have declared national emergencies since World War II. As The Washington Post previously reported, Bill Clinton declared emergencies 17 times, George W. Bush 12 and Barack Obama 13.

The vast majority have been economic sanctions against foreign actors whose activities pose a national threat, according to Elizabeth Goitein, co-director of the Brennan Center for Justice’s Liberty and National Security Program.

A handful others have involved noneconomic crises:

Clinton declared a national emergency during the 1996 Cuba embargo, preventing U.S. ships or aircraft from entering Cuban territory without authorization. Obama declared a national emergency during the H1N1 Swine Flu epidemic in 2009 to activate disaster plans to set up proper patient treatment.

Bush declared a national emergency after the terrorist attacks of Sept. 11, 2001; the order is still in effect today.

Is a national emergency the same thing as an executive order?

In general, national emergencies have been declared through executive orders.

An executive order is a command issued by the president that carries the force of law. The power is authorized, in part, by Article II of the U.S. Constitution.

Executive orders direct federal agencies on how to spend available resources.

Typically, Dickinson explained, they’re used by a president to bypass Congress and force certain departments to act. They are legally binding and need not be approved by Congress, though they can be reviewed and overturned by a court.

Thousands have been created by past presidents, covering topics as varied as the duties of the commander in chief.

For example, President Franklin Roosevelt issued an executive order in 1942, leading to Japanese American internment camps during World War II, and President Dwight Eisenhower issued an executive order in 1957, dispatching federal troops to aid with desegregation in Little Rock. In 2014, an Obama-issued order halted the deportation of more than 4 million undocumented immigrants.

The U.S. Supreme Court has only rarely held an executive order invalid — one was issued by Harry S. Truman in 1952, seizing the country’s steel mills during the Korean War, and another came from Clinton in 1995 and involved workers on strike.

Executive orders do not create new law or allocate additional funding, which is where Trump has run up against congressional hurdles.

Following his inauguration, Trump issued an executive order making construction of a barrier wall across the southwest border a federal priority. The wall, of course, could not be built unless Congress provided him with the funds.

What happens once a national emergency is declared?

If Trump follows through on declaring a national emergency, what comes next is legal and legislative pushback.

Even though there aren’t many limits on a president’s ability to declare an emergency, it does not create carte blanche freedom to act.

Anyone directly affected by the order can challenge it in court, which Goitein said will almost certainly happen here.

Congress can also draft a concurrent resolution to terminate the state of emergency, leading to a somewhat novel issue. Ordinarily, congressional resolutions support a president’s declaration of a national emergency. In the current scenario, it’s unclear whether Congress will be able to cancel the emergency order without the president’s sign-off.

Trump’s battle over the border wall has brought out the need to have a definition of — or at the very least, broad parameters for — what qualifies as a “national emergency,” according to Goitein.

Congress passed the National Emergencies Act to give presidents access to tools they believed necessary.

"If Trump is doing something that Congress has either prohibited or expressly did not allow, he’s doing something wrong,” she said, noting that Congress will probably consider limiting the presidential abilities under the National Emergencies Act. “Before this comes up again, let’s look at the act and figure out where we need to build in more guardrails.”